Polishing without touching – how to reduce roughness in refractive micro-optical elements

Time 2017-11-10 09:30~09:50 Place Convention Hall Ⅰ
Code No. Session Chair
Name Dr. Helmut Schift
Affiliation Paul Scherrer Institut (PSI)
Title Polishing without touching – how to reduce roughness in refractive micro-optical elements
First nameRobert
Last nameKirchner
AffiliationTU Dresden
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First nameNachiappan
Last nameChidambaram
AffiliationPaul Scherrer Institut (PSI)
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First nameMirco
Last nameAltana
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Refractive micro-optical elements are finding applications in high volume consumer electronic devices. A truly 3D capable approach with higher resolutions in lateral and vertical direction is two-photon polymerization (2PP). The trajectories and layering during the DLW often result in roughness in the range of the writing increment, which has adverse effects for optical applications. If a surface has to be subsequently smoothed out by post-processing, it must be achieved using a method that only modifies the surface and does not change the structural details or the overall shape. For this a method known as TASTE was developed. It involves selectively changing the material properties (glass transition temperature) of the part of the sample that needs modification, i.e., locally confined in lateral or vertical direction. We therefore copied the original structure (master) into a PMMA structure by thermal imprint. By using 172 nm UV exposure, the PMMA was modified at a defined depth of the surface by chain scission. This was followed by molecular weight dependent reflow at elevated temperatures. Using our polishing process, a roughness of 100 nm could be removed while preserving the concave shape with up to 50 μm high structures and deep central sags surrounded by high aspect ratio tips between adjacent lenses in array configuration. This was also confirmed optically for a microlens array with several 100 microlenses.